Abstract
Desert dust modelling and forecasting attract growing interest, due to the numerous impacts of dusts on climate, numerical weather prediction, health, ecosystems, transportation, as well as on many industrial activities. The validation of numerical tools is a very important activity in this context, and we present here an example of such an effort, combining in situ (horizontal visibility in SYNOP messages, IMPROVE database) and remote-sensing data (satellite imagery, AERONET aerosol optical thickness data). Interestingly, these measurements are available routinely, and not only in the context of dedicated measurements campaign; thus, they can be used in an operational context to monitor the performances of operational forecasting systems. MOCAGE is the chemistry-transport model of Météo-France, used operationally to forecast the three-dimensional transport of dusts and their deposition. Two very long-range transport episodes of dust have been studied: one case of Saharan dust transported to East America through Asia and Pacific observed in November 2004 and one case of Saharan dust transported from West Africa to Caribbean Islands in May 2007. Episodes of geographical extension had seldom been studied, and they provide a very selective reference to compare the modelled desert dusts with. The representation of dusts in MOCAGE appears to be realistic in these two very different cases. In turn, the model simulations are used to make the link between the complementary information provided by the different measurements tools, providing a fully consistent picture of the entire episodes. The evolution of the aerosol size distribution during the episodes has also been studied. With no surprise, our study underlines that deposition processes are very sensitive to the size of dust particles. If the atmospheric cycle, in terms of mass, is very much under the influence of larger particles (some micrometres and above), only the finer particles actually travel over thousands of kilometres. This illustrates the need for an accurate representation of size distributions for this aerosol component in numerical models and advocates for using a size-resolved (bin) approach as sinks, and particularly, deposition do not affect the emitted log-normal distributions symmetrically on both sides of the median diameter. Overall, the results presented in this study provide an evaluation of Météo-France operational dust forecasting system MOCAGE.
Highlights
Dust emitted from desert surfaces by wind erosion of soil can be transported in the atmosphere over thousands of kilometres (i.e. Asian dust over the Pacific Ocean, Gong et al, 2006; Zhao et al, 2006 or African dust over the Atlantic Ocean, Prospero and Carlson, 1972; Colarco et al, 2003a,b)
MOCAGE dynamics are driven in this study by meterological analysis coming from Meteo-France’s operational global model ARPEGE
In MOCAGE, the extinction coefficient is calculated from the Mie and the complex refractive index m of desert dust is m = n + ik = 1.53 − 0.0078i, (11)
Summary
Dust emitted from desert surfaces by wind erosion of soil can be transported in the atmosphere over thousands of kilometres (i.e. Asian dust over the Pacific Ocean, Gong et al, 2006; Zhao et al, 2006 or African dust over the Atlantic Ocean, Prospero and Carlson, 1972; Colarco et al, 2003a,b). Transport is computed by a semi-Lagrangian scheme for all species (Josse et al, 2004), turbulent diffusion is accounted for following (Louis, 1979) and MOCAGE convection is parametrized with a mass flux scheme, described in Bechtold et al (2001) It can represents both gaseous and aerosols species simultaneously, but the present work relies only on the aerosol part of the model. Studying the emissions modelled by MOCAGE for the meteorological year 2000, major known areas of emissions appearat Mauritania, Bodele depression, south of Egypt and south of the Arabic peninsula. In all these areas, there are more than 200 d of emissions (Fig. 1a). This phenomenon leads to the interseasonal cycle of dust emissions over North Africa
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